This invention relates to switching-mode DC-to-DC converters, and more particularly to those of the dual-mode variety capable of driving, or rapidly turning on and off, the active switch constantly under normal load and at temporarily enforced intervals under light load.
A typical conventional switching-mode DC-to-DC converter includes a transformer having a primary winding coupled to the pair of DC input terminals via an active switch, and a secondary winding coupled to the pair of DC output terminals via a rectifying and smoothing circuit. The active switch is driven by a switch control circuit which is powered from a tertiary winding of the transformer via another rectifying and smoothing circuit.
It has been suggested in conjunction with the DC-to-DC converter of the kind outlined above to drive the active switch at intervals when the power requirement of the load is low, in order to reduce switching loss and hence to enhance the efficiency of the converter. The intermittent driving of the active switch in light load mode makes the switchings drastically less in number on an average per unit length of time than when the switch is driven continuously in normal load mode. The diminution of average switchings in light load mode leads to the curtailment of switching loss and consequently to a higher overall efficiency of the converter.
Such dual-mode driving of the voltage regulator switch has had a weakness left undefeated, however. The voltage across the smoothing capacitor of the first recited rectifying and smoothing circuit, on the output side of the transformer, rises when the active switch is being driven, and diminishes when it is not. The same applies to the supply voltage (herein termed the control voltage) fed from the second rectifying and smoothing circuit coupled to the transformer tertiary to the switch control circuit for powering the latter.
A problem arose because of a difference in the rate of voltage diminution between the capacitor of the first rectifying and smoothing circuit and that of the second rectifying and smoothing circuit when the power requirement of the load was extremely low. The power requirement of the switch control circuit is hardly affected by changes in the power requirement of the load, so that the capacitor voltage of the second rectifying and smoothing circuit declines far more greatly than that of the first rectifying and smoothing circuit while the active switch is not being driven during operation in light load mode. Actually, the switch control circuit went out of operation, becoming totally incapable of driving the active switch, in the worst case where the control voltage being supplied to the switch control circuit fell below the allowable minimum.
Once set out of operation, the switch control circuit must return to normal functioning after a mandatory restart period of several hundreds milliseconds. The smoothing capacitor was left uncharged during this restart period, so that its voltage dwindled further, making it impossible or difficult to feed the load as required.
It might be contemplated for the mitigation of the foregoing difficulty to provide a transformer tertiary of a greater number of turns, and a smoothing capacitor of greater capacitance, than heretofore. Such a transformer would introduce a greater power loss, running counter to the prime objective of overall converter efficiency enhancement. Another possible measure might be to drive the active switch at shorter intervals throughout the light load mode. This remedy is objectionable because it would make the average number of switchings greater per unit length of time, again to the impairment of the noted prime objective.
A more drastic approach is suggested by Japanese Unexamined Patent Publication No. 2003-33018, which calls for invalidation of the light load mode altogether in the event of an excessive drop in the control voltage supplied to the switch control circuit. The active switch is driven continuously even when it should be at intervals, thereby giving rise to as much switching loss under light load as under normal load.